The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method for performing a track to track seek in a disc drive.
One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within the slider. The slider is a small ceramic block which is passed over the disc in a transducing relationship with the disk. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disk. Most sliders have an air bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disk. At the same time, the air rushing past the depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular fly height. The fly height is the thickness of the air lubrication film or the distance between the disc surface and the transducing head. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disk.
Information representative of data is stored on the surface of the memory disk. Disc drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is moved by a voice coil motor (xe2x80x9cVCMxe2x80x9d). Attached to the actuator arm of the actuator assembly is a coil, most commonly known as a voice coil. The voice coil is one of the major portions of the VCM. Magnets attached to the base of the disc drive form the other major portion of the VCM. By controlling the amount and direction of the current passing through the voice coil, the direction and the speed at which the actuator arm can be moved is regulated. Of course, the voice coil does have physical constraints one of which is a maximum current which can be passed through the voice coil for a given supply voltage. When maximum current is passed through the voice coil this is called operating in saturation mode. The amount of current applied to the voice coil during saturation mode is referred to as the saturation current.
When the actuator assembly moves the read/write head from a beginning track radially across the other tracks to a selected target track this is referred to as a seek. Many times seeks are to adjacent tracks. Other times the seeks can span up to 2000-3000 tracks or even more depending on the track density of the drive. Actuator assemblies within disc drives generally accelerate and decelerate during long seeks to keep access times to a minimum. Generally the actuator assemblies follow what is known as a velocity profile during a seek to move the read/write head from a start track to a target track. A velocity profile is a preprogrammed equation or table which lists a desired velocity verses the stopping distance remaining until reaching the target track. The profile velocity value is the highest possible value of velocity the actuator can have at a particular distance so that the actuator can still be decelerated to a stop upon reaching the target track. The amount of deceleration that can be applied to the actuator is a function of many variables including voice coil resistance, file torque constant and power supply voltage. These variables are generally not known for each specific disc drive and as a result, the velocity profile is designed using worst case values to assure that there will always be adequate deceleration capability to stop the read/write head actuator upon reaching the target track.
A typical seek is accomplished by calculating distance left to go to the target track, selecting the velocity from the velocity profile which corresponds to the calculated distance to go, determining the actual actuator velocity and subtracting the actual actuator velocity from the selected velocity obtained from the velocity profile. This value is then multiplied by a gain to give a control current output to the voice coil. The acceleration produced is a function of many factors. Amongst the major factors are the magnetic flux density of the VCM and the environmental operating conditions, such as temperature, of the drive. These parameters vary from disc drive to disc drive.
During the design process of velocity or seek mode, a fixed acceleration constant is used as a reference point to determine optimized seek trajectories resulting in characterized move times and settle times to the target track center for a given seek length. If the acceleration constant deviates away from the reference point chosen during the design phase, seek and settle time characteristics will change. This results in unpredictable move times and will cause either overshooting or undershooting on the position error signal during the settle phase of the seek. At worst case, the seek may not complete successfully at the target track resulting in a seek error. A force constant is a multiplier associated with the gain for the VCM drive current that equalizes the results amongst the disc drives.
A standard technique used throughout the industry is to maintain a fixed acceleration/deceleration for a fixed demand. The way this is achieved is to scale the output current by multiplying the demand by a term called the force constant. For example, if the acceleration constant for a drive is 10% less than the design reference point at 1.0 amps flowing through the VCM, the force constant parameter will increase the VCM current by 10% to 1.1 amps. This will ensure that the accelerations required at the design stage will be met under all conditions. The force constant either reduces or increase the output current to the VCM so that the actual velocity profile of a drive more closely corresponds to the designed velocity profile and so that when a velocity demand is made on a drive, the drive will behave as designed.
Typically, the force constant is calibrated in one of two ways. The first way is to run force constant calibration routines at selected time intervals. The problem associated with this technique is that the input/output interruptions occur which the customer sees as poor performance in the disc drive or the computer in which the disc drive is housed. The second way of calibrating the force constant is in real time. In the past, force constant calibrations in real time were calculated during the deceleration portion of a long seek. For example, the force constant calibrations were made only on seeks of 1000 tracks or more. If a disc drive makes predominantly short seeks, the parameters that affect the force constant may change. The force constant parameter would not change since no long seek would be encountered. The result is that force constant parameter would not be correct and seeks and settle times would be either too short or too long. Seek errors could result.
What is needed is a method for calibrating the force constant for a VCM motor that does not effect the operations of the I/O. What is also needed is a method that can be used to produce real time updates to the force constant so that the force constant is corrected and tracks the changes in the parameters that effect the force constant. The method should also provide for ease of manufacture so that a new system hardware configuration is not needed. The method must also provide for a more robust disc drive which is rugged and stable over time. In other words, the system must be able to last for the life of the drive and provide for a drive that is easier to use as well as being more dependable.
An information handling system, such as a disc drive, includes a base, a disc stack rotatably attached to the base, and an actuator assembly movably attached to the base. Attached to one end of the actuator assembly is one or more transducers. Attached to the other end of the actuator is a voice coil which forms a portion of a voice coil motor. Magnets attached to the base form the other portion of the voice coil motor. A current driver for the voice coil delivers an amount of current to the voice coil to move the actuator assembly. A force constant is a multiplier applied to the amplifier so that the output current to the voice coil produces a velocity output replicating the designed velocity output. The force constant is determined during the acceleration phase of the movement of the actuator and transducer. A velocity error signal is produced by comparing the actual velocity of the actuator and transducer to the demand velocity. The error signal can be based on one point in time or over a period of time. The selected time is during a linear mode of the current driver.
Also disclosed is a method for calibrating the force constant of a voice coil motor of a disc drive. The voice coil motor is for accelerating and decelerating an actuator. The disc drive includes a power amp driver for providing current to the voice coil motor. The method includes applying current to the voice coil so that the actuator assembly follows a velocity demand profile for a selected portion of the acceleration phase. This accelerates the actuator. The velocity summation error over the selected portion of acceleration time is determined. The velocity summation error over the selected portion of acceleration time is t hen compared to a reference value. The force constant is then determined based on the comparison between the velocity summation error over the selected portion of acceleration time to a reference value.
In one embodiment, the selected portion of the acceleration phase includes a linear mode of the power amplifier or driver. In another embodiment, the velocity error over the selected portion of acceleration time includes integrating the velocity error over the selected time period. The selected time can also be a fixed amount of time or associated with the time required to move the actuator assembly a fixed distance during a seek. In some instances, the velocity error over the selected portion of acceleration time includes comparing the actual velocity to the velocity demand at a single point to determine the velocity error.
Advantageously, the method described for calibrating the force constant for a VCM motor that does not effect the operations of the I/O. Force constants can be calibrated in real time over relatively short seeks to produce real time updates to correct the force constant before the changes in the parameters that effect the force constant cause the value to drift out of calibration. The method requires no new system hardware and can be implemented as a software change to the microcode used to run the disc drive. Since the force constant is less likely to drift out of calibration, the method provides for a more robustly designed disc drive which is more stable over time. The calibration of the force constant lasts over the life of the drive and provides for calibration of the force constant as the electronic components drift during their lives. The disc drive is thus more dependable over its life.